Disk drive having improved timing marks

ABSTRACT

A disk drive has timing marks (TMs) on the disk, that are chosen to reduce the probability of misidentification of a TM in the presence of read errors. The disk drive searches for TMs within a fixed TM search window which extends past the TM on the disk. A TM preferably maximizes the post-shift sliding distance for m post-shifts of the TM pattern, where m corresponds to the TM search window boundary. In this manner, the probability of a misidentification of the TM due to a post-shift having a small distance from the TM pattern is reduced. The TM pattern also provides pre-shift error resistance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to copending, commonly assigned applicationdocket number HSJ9-2003-0240US1, entitled “Method for operating diskdrive having improved timing marks”, filed on even date herewith, andhereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to disk drives for data storage, and inparticular to disk drives that have timing marks on disk tracks forservo head control or for data synchronization.

BACKGROUND

Data is recorded on magnetic disk drives in radially spaced tracks onthe surface of one or more rotating disks. Typically, a single recordinghead, which may be an inductive read/write head, or an inductive writehead in combination with a magnetoresistive read head, is associatedwith a corresponding magnetic recording surface of each disk. Eachrecording head is moved by an actuator in a generally radial directiontoward and away from the center of rotation of the disks, to align thehead to a desired track.

It is necessary to know the precise radial and circumferential locationof the recording heads relative to their associated disk surfaces. Forconventional fixed-block architecture disk drives, position informationis typically recorded onto the disk as servo information in angularlyspaced servo sectors interspersed among the data sectors.

Each of the servo sectors contains a servo timing mark (STM), which is adefined bit pattern. A servo timing mark is also known as a servoidentification (SID) or a servo address mark (SAM) in the art. When anSTM is identified in reading the disk subsequent detection of servoinformation (e.g., track identification and position error signalbursts) is initiated. This servo information is used by servoelectronics to determine the radial position of the head and to providefeedback to the actuator to ensure the head remains positioned over thecenterline of the desired track. In some cases, the STMs are also usedto assist in locating specific data sectors where user data is to beread or written (e.g., as described in U.S. Pat. No. 5,500,848).

Accurate detection of STMs is crucial to proper disk drive operation,since such detection is required in order to correctly recognizesubsequent servo information. If a servo sector is not recognized due tofailure to detect the STM, the servo electronics will rely on lessrecent servo information (e.g., from the most recent recognized servosector), and servo tracking and timing accuracy will be diminished.Also, if an STM is incorrectly detected at the wrong location,subsequent servo information will be missed and/or incorrectlyinterpreted and acted upon.

Incorrectly recognizing an STM and acting incorrectly as a result tendsto be more problematic than failure to recognize an STM.

Detection of an STM pattern within a servo sector can be reformulated asdetection of a known bit pattern (i.e., the STM bit pattern) embeddedwithin an input stream subject to error (i.e., the sequence of bits readfrom the disk). A known approach for locating the STM bit pattern withinthe input stream is to compare an n-bit STM bit pattern to a windowconsisting of n consecutive bits in the input stream, and successivelyshifting the window, one bit at a time, until a match is found betweenthe window and the STM bit pattern.

How well two bit sequences of equal length match each other isconveniently described by the Hamming distance, which is the number ofbits which differ in the two sequences. A perfect match corresponds to aHamming distance of zero. It is also convenient to define the aboveshifts as pre-shifts, where pre-shift 1 corresponds to a window positionstarting 1 bit before the start of the STM pattern in the input stream,pre-shift 2 corresponds to a window starting 2 bits before the STMpattern, etc. Accordingly the above procedure can be expressed ascalculating the Hamming distance for pre-shifts k, k−1, k−2, . . . , 2,1, 0 in succession, for a suitably chosen integer k, and identifyingpre-shift 0 (i.e., the STM) when a Hamming distance of 0 is found.

In the absence of errors, a perfect match will occur between the STMpattern and the window when the window is aligned to the STM pattern inthe input stream, and this perfect match indicates detection of the STMon the disk, provided there is no other perfect match to the STM patternin the input stream. Naturally, it is necessary to restrict the searchfor an STM to regions of the disk which are substantially free of userdata, since user data may coincidentally contain the same bit pattern asthe STM bit pattern.

In the presence of errors, a perfect match is not to be expected, evenwhen the window is aligned to the STM pattern in the input stream.However, it is known in the art to select an STM pattern that providesgenerally large Hamming distance for pre-shifts 1 through n for an n-bitSTM pattern. Such an STM pattern minimizes the chances ofmisidentification of the STM pattern based on a low Hamming distancebetween the STM pattern and a pre-shifted position of the window. Sincethe pre-shifted window includes bits from the input stream before theSTM pattern, a preamble bit sequence having at least n bits is typicallyprepended to the STM pattern on the disk. Inclusion of the preamblefixes the pre-shifted bit patterns, and renders them independent of anydata or other information on the disk. A commonly used preamble patternis all ones, but any other bit pattern may be used as well. If theminimum Hamming distance between the STM pattern and pre-shifts 1through n is d1, then d1 is referred to as the pre-shift slidingdistance.

However, this prior art approach has some drawbacks. Essentially, asearch for the presence of an STM within an STM search window on thedisk is performed, since this search cannot go on indefinitely. If theSTM is not found within the search window, some alternative correctionaction is taken. If the STM search window does not extend past the STM,then the probability of missing the STM is undesirably increased (e.g.,if the STM window is set incorrectly by only 1 bit so that it does notinclude the entire STM, the STM will be missed in the search). However,if the STM search window extends past the STM, then erroneousrecognition of the STM is possible, because the prior art STM patternsare optimized only with respect to pre-shifts. Erroneous recognition ofan STM pattern is typically a much more severe problem than not findingan STM pattern, since erroneous recognition may lead the disk drive totake further erroneous decisions (e.g., misidentifying the track).

Accordingly, it would be an advance in the art to provide an STM searchwindow which extends past the STM on the disk while avoiding theerroneous recognition problem identified above. It would also be anadvance to provide STM patterns compatible with this STM search window.

SUMMARY

In one aspect of the invention, the present invention provides a diskdrive having timing marks (TMs) within timing sections of disk tracks,with reduced probability of missing or misidentifying TMs in operation.More specifically, the disk drive searches for TMs on the disk within anTM search window which extends past the TM on the disk, which reducesthe probability of missing an TM due to misalignment of the TM searchwindow with respect to the TM.

TM patterns for use with embodiments of the present invention arepreferably chosen to reduce the probability of misidentification of anTM in the presence of read errors. More specifically, an n-bit TM ispreferably chosen to maintain a maximal post-shift sliding distance d2for m post-shifts of the TM pattern relative to the input stream, wherem is selected to correspond to the nominal boundary of the TM searchwindow. In this manner, the probability of a misidentification of the TMdue to a post-shift having a small Hamming distance from the TM patternmay be reduced. In addition, the TM pattern maintains a selectedpre-shift sliding distance d1 for n pre-shifts of the TM patternrelative to the input stream.

In a preferred embodiment, the timing marks are-servo timing marks(STMs), and are followed by servo position information. Recognition ofthe STMs enables the servo position information to be used to controlthe head position during disk operation. In an alternate preferredembodiment, the timing marks are data timing marks (DTM), and arefollowed by data. Recognition of the DTMs facilitates bytesynchronization of data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a disk drive according to one embodiment ofthe invention.

FIG. 2 a schematically shows a portion of a track from a disk driveshowing a servo sector and a data sector, according to an embodiment ofthe invention.

FIG. 2 b schematically shows details of the servo sector of FIG. 2 a.

FIG. 3 is a table showing Hamming distances from a pattern u1={0, 0, 0,1, 0, 0, 1} to pre-shifted and post-shifted instances of u1, using apreamble pattern of all ones, according to an embodiment of theinvention.

FIG. 4 is a table showing Hamming distances from a pattern u2={0, 0, 1,1, 1, 0, 1} to pre-shifted and post-shifted instances of u2, using apreamble pattern of all ones, according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a disk drive 10 that is representative of thetype of disk drive according to one embodiment of the invention. Diskdrive 10 includes disk 12, read/write head 16 (also called a datarecording transducer), actuator 18, TM decoder 20, and servo electronics22. Disk 12 has a set of radially spaced tracks, one of which is shownat 14. Disk 12 is rotated about its center 13 by a motor (not shown).Information is written to and/or read from track 14 by read/write head16. Read/write head 16 is connected to actuator 18, which radiallypositions head 16 over a selected track. For example, actuator 18 may berotated about axis 19 by an actuator motor (not shown).

FIG. 2 a schematically shows a portion of track 14. Track 14 includesservo sectors (SS) interspersed with data sectors (DS). A servo sectoris shown as 30 in FIG. 2 a, and a data sector is shown as 32 in FIG. 2a. FIG. 2 b shows a more detailed view of servo sector 30 of FIG. 2 a.Servo sector 30 includes a preamble bit pattern (P), shown as 34, aservo timing mark bit pattern (STM), shown as 36, and servo information(SI) shown as 38. In FIG. 2 b, the head encounters regions 34, 36, and38 in that order (i.e., the tracks shown in FIGS. 2 a and 2 b are readfrom left to right).

Returning now to FIG. 1, read/write head 16 is operably connected to TMdecoder 20, as indicated by line 24. As track 14 passes under head 16,the head will encounter servo sectors 30 and data sectors 32. TM decoder20 receives a bit stream corresponding to the bit stream on track 14,and functions to detect the STM bit pattern 36 within servo sectors 30as they pass under the head. Upon identification of an STM bit pattern36, TM decoder 20 transmits an “STM found” signal to servo electronics22, as schematically indicated by line 26 in FIG. 1. Servo electronics22 also receives servo information 38 from TM decoder 20, and makes useof servo information 38, gated by the “STM found” signal, to performclosed loop control of actuator 18, schematically indicated by line 28,such that head 16 is centered over a desired track (i.e., track 14 inthis example). Further details of head control in response to servotiming marks 36 and servo information 38 are given in U.S. Pat. No.5,903,410, incorporated by reference in its entirety.

In order to reduce the probability of misidentification of an STM, TMdecoder 20 usually only looks for STMs within a STM search window, whichnominally extends several bits past an STM on the disk and at least nbits before an STM on the disk. In normal disk drive operation,information obtained from the previous servo sector encountered willenable the STM search window to be set with sufficient accuracy todefine its nominal boundaries with respect to the next STM expected asindicated above. Unusual events, such as track changes, error recoveryand disk drive start-up use other methods for bounding the search forthe STMs, which are known in the art.

Typically, TM decoder 20 compares STM pattern 36 to a window ofconsecutive bits read from servo sector 30, and this window issuccessively stepped through the bit stream read from servo sector 30.For example, reading the bit stream from servo sector 30 into a shiftregister is one way to provide this stepped windowing.

Therefore, the performance of an STM pattern u1 can be assessed bycomputing Hamming distances between u1 and shifted versions of u1, asshown on FIG. 3 for u1={0, 0, 0, 1, 0, 0, 1}. The top row of FIG. 3shows the sequence of bits on the disk, also referred to as the inputstream. In this example, the STM pattern u1 is preceded by a preamblepattern which is all ones. Pre-shift k, indicated by “pre k” on FIG. 3,indicates a window position that is shifted k bits before the STMpattern in the input stream. Similarly, post-shift k, indicated by “postk” on FIG. 3, indicates a window position that is shifted k bits afterthe STM pattern in the input stream.

Due to the presence of the preamble pattern in the example of FIG. 3,the Hamming distances between u1 and each of the pre-shifts can becomputed straightforwardly, and are given in the “distance” column ofFIG. 3. In this example, the smallest Hamming distance between u1 andany of pre-shift 1 through pre-shift n, also known as the pre-shiftsliding distance, is 4. Here n is the number of bits in the STM pattern.When the window is aligned to the STM pattern, indicated by “align” onFIG. 3, the Hamming distance between u1 and the window is zero, asindicated on FIG. 3.

FIG. 3 also shows the first few post-shifts. For post-shifts, the windowextends past the end of the STM pattern and covers bits which are notpart of the STM bit pattern. It is possible to add a postscript bitpattern after the STM, similar to the preamble bit pattern before theSTM, to make the Hamming distance calculation definite for post-shifts.Alternatively, it is also possible to perform the distance calculationfor post-shifts as a “worst-case” calculation, where all bits of thewindow extending past the STM pattern on the disk are assumed to matchthe STM pattern. The worst case approach has the advantage that fewerbits on the disk are required for STM detection than if a postscriptpattern is used (which increases the fraction of disk area devoted touser data), while the use of a postscript pattern will provide improvederror resistance. The post-shift distances given in Example 3 arecalculated using the above worst case assumption.

The STM pattern of FIG. 3 provides a pre-shift sliding distance of 4,and a post-shift sliding distance of 3 for two post-shifts pastalignment. Since the number of post-shifts past alignment which are ofinterest is a variable parameter, the post-shift sliding distance isexpressed with the notation (d2; m), meaning that distance no less thand2 is obtained for the first m post-shifts past alignment. With thisnotation, the worst case post-shift sliding distance is the minimumdistance between the first n-k bits of the STM bit pattern and the lastn-k bits of the STM bit pattern as k is varied from 1 to m. We havefound that the pre-shift sliding distance and the post-shift slidingdistance can be traded off. For example, by reducing a requirement onpre-shift sliding distance, it is usually possible to increase m for afixed d2 in the post-shift sliding distance (d2, m).

FIG. 4 provides a demonstration of such a trade. The STM sequence inFIG. 4 is u2={0, 0, 1, 1, 1, 0, 1}. As seen in FIG. 4, this sequenceprovides a pre-shift sliding distance of 3 (one less than provided by u1of FIG. 3), and provides a post-shift sliding distance of (3; 3) asopposed to the post-shift sliding distance of (3; 2) provided by u1 ofFIG. 3.

STM patterns having desirable pre-shift and post-shift distances can bedetermined using a systematic computer-assisted search. Results of sucha search for 16-bit STM patterns are presented in the following table:TABLE 1 Error-resistant 16-bit STM patterns pre-shift post-shift slidingsliding distance pattern distance dl (d2; m) 0001 0100 1001 1101 9(9; 1) 0001 0100 1001 1101 9 (8; 1) 0001 0100 1001 1101 9 (7; 2) 00010100 1001 1101 9 (6; 4) 0001 1001 0101 0011 8 (8; 3) 0010 0010 1100 00118 (7; 5) 0000 0111 0101 0011 8 (6; 7) 0000 0101 1011 1001 8 (5; 9) 00100111 0100 0011 7 (7; 6) 0000 0101 1011 1000 7 (6; 8) 0000 0101 1011 11007  (5; 10) 0100 0111 1101 1000 6 (6; 9) 0000 0101 1011 1100 6  (5; 10)0000 0001 0100 1111 6  (4; 12)

In Table 1, the STM patterns are given in the leftmost column. Spacesare inserted into the patterns as shown in Table 1 for readability only.On the disk, these STM patterns occupy 16 consecutive bit positions. Foreach case (i.e., row of Table 1), the pre-shift sliding distance d1given in the center column, and the post-shift sliding distance (d2; m)is given in the right column. In Table 1, post-shift sliding distancesare based on the worst case calculation discussed above. The patterns inTable 1 are selected to have maximal m, given n, d1, and d2. Generally,decreasing d1 and/or d2 allows this maximal m to be increased to alarger value, as shown on Table 1, and as expected from the examples ofFIGS. 3 and 4. The results of Table 1 are obtained using a preamblepattern of all ones. If a different preamble pattern is used, equivalentperformance will be obtained (i.e., m in each row will not change), butdifferent STM patterns will be determined by the computer search forachieving this performance.

Although the specific optimization performed to generate Table 1 is themaximization of m given d1, d2, and n, results of other optimizationsmay also be determined from Table 1. Two examples are as follows: forn=16, d1=9, m=2, the maximal d2 is 7; and for n=16, d2=7, m=5, themaximal d1 is 8. Alternatively, the systematic computer search used togenerate Table 1 can readily be altered to maximize d1 given the otherparameters, or to maximize d2 given the other parameters, or to minimizen given the other parameters as opposed to maximizing m given the otherparameters. All four approaches are substantially equivalent ways togenerate STM patterns in accordance with teachings of the invention. Itis convenient to refer to STM patterns generated by any of the aboveidentified optimization methods as (n, d1, d2, m) patterns.

In practice, it is expected that the most typical optimization will bethe maximization of d2, given n, d1 and m. The reason for this is that nis typically fixed by high level design considerations (e.g., how muchdisk area to devote to the STM patterns), d1 is selected to provide thedesired level of error resistance for pre-shifts, m and the boundary ofthe STM search window are selected together to reduce the probability ofthe search window boundary falling within the STM to a design value, andthen d2 is maximized to provide as much post-shift error resistance aspossible given the other constraints. Providing resistance to pre-shifterrors tends to be more important than providing post-shift errorresistance. One reason is that post-shift error resistance is onlyrelevant if the STM is missed, which itself is a low probability event.Accordingly, a design will typically end up with d1>d2, although this isnot required to practice the invention.

In some cases, the computer search for optimized STM patterns willprovide multiple patterns satisfying the same constraint. In thesecases, one can select an STM pattern from a set of (n, d1, d2, m)patterns having the same n, d1, d2, and m according to various criteria.Such criteria include, but are not limited to, maximizing the number ofones, minimizing the length of the longest run of zeros, and minimizingthe number of times the distance takes on its minimal value forpre-shifts and/or post-shifts.

The examples of FIGS. 3 and 4, and the results of Table 1, are all basedon the use of a preamble sequence of at least n bits, where all of thepreamble bits are ones. Any other fixed preamble bit sequence of atleast n bits is also suitable for practicing the invention. In practice,other criteria may be used to select the preamble bit pattern, such assuitability for automatic gain control.

In the examples of FIGS. 3 and 4, and the results of Table 1, all of theHamming distances were computed bitwise. This method of computingdistances is suitable in cases where bit errors tend to be independent.In some cases, however, bit errors are not independent (e.g., if errorstend to occur in bursts affecting some number of consecutive bits). Insuch cases, the use of a burst Hamming distance (instead of a bitwiseHamming distance) to select and use STM patterns in accordance with theinvention is suitable. The j-bit burst Hamming distance between two bitsequences is calculated by comparing the two sequences j bits at a time,according to the following procedure: If two j-bit subsequences areidentical, add 0 to the distance. If two j-bit subsequences differ, add1 to the distance. Repeat for each j-bit subsequence in the twosequences. If there is a last subsequence of less than j bits, treat itthe same way as all the other subsequences.

Thus the j-bit burst Hamming distance between two bit sequences is thenumber of differences between the two sequences, where differences arecounted by comparing j-bit blocks as opposed to single bits. Accordingto this definition, the 1-bit burst Hamming distance is identical to thebitwise Hamming distance. STM patterns according to the presentinvention can be generated by systematic computer searching using eithera bitwise Hamming distance, or a j-bit burst Hamming distance.

Although the embodiment discussed above is presented in the context of amagnetic disk drive having a specific architecture, the invention isapplicable to magnetic or optical disks having other architecturesand/or other servo techniques which depend on recognition of a known bitpattern on a disk.

Furthermore, although the embodiments discussed in detail above relateto locating a servo timing mark to signal the presence of servoinformation on a disk, the invention is applicable to locating any markrepresented by a bit pattern on a disk. For example, bytesynchronization is required before reading data from a disk, andtherefore a known data timing mark bit pattern typically precedes thedata. The apparatus and methods of the present invention areadvantageously applicable for locating such data timing marks withreduced probability of error. For both applications (servo and data), itis necessary to locate a timing mark within a timing section within adisk track. For the servo application, the timing sections are the servosectors identified above, while for the data application, the timingsections precede the data.

1. A disk drive comprising: a) a rotatable disk having a plurality oftracks, each track having a plurality of timing sections comprising: i)a preamble represented as a preamble pattern of at least n bits; and ii)a timing mark (TM) following said preamble, said TM being represented asa TM pattern of n bits, wherein said TM pattern has a pre-shift slidingdistance d1 to the concatenation of said preamble pattern with said TMpattern, and has a post-shift sliding distance (d2; m) to said TMpattern, said TM pattern being referred to as a (n, d1, d2, m) pattern,wherein said TM pattern satisfies an optimality condition selected fromthe group consisting of: m is maximal given n, d1, and d2; d1 is maximalgiven n, d2, and m; d2 is maximal given n, d1, and m; and n is minimalgiven d1, d2, and m; b) a read/write head for reading information fromsaid disk and/or writing information to said disk; and c) a TM decoder,responsive to information read from said disk by said head, fordetecting the TM patterns in said read information to thereby signal thepresence of the timing sections, wherein said TM decoder searches forsaid TM patterns within a TM search window which nominally extends mbits past the last bit of the TM on the disk and at least n bits beforethe first bit of the TM on the disk.
 2. The disk drive of claim 1,wherein said timing marks are followed by data on said tracks.
 3. Thedisk drive of claim 1, wherein said timing marks are followed by servoposition information on said tracks.
 4. The disk drive of claim 3,further comprising: d) an actuator connected to said head forpositioning said head to one of said tracks and maintaining said head onsaid one of said tracks; and e) servo electronics coupled to said TMdecoder for controlling the actuator in response to said servo positioninformation read by said head after detection of said TMs by said TMdecoder.
 5. The disk drive of claim 1, wherein the sliding distances d1and (d2; m) are bitwise Hamming distances.
 6. The disk drive of claim 1,wherein the sliding distances d1 and (d2; m) are j-bit burst Hammingdistances.
 7. The disk drive of claim 1, wherein said TM pattern is amember of a set of (n, d1, d2, m) patterns, wherein all members of saidset have the same n, the same d1, the same d2 and the same m, said sethaving at least two members.
 8. The disk drive of claim 7, wherein eachmember j of said set has a longest run of zeros with length L(j), andsaid TM pattern is a member of said set with minimal L(j).
 9. The diskdrive of claim 7, wherein said TM pattern is a member of said set havinga maximal number of ones.
 10. The disk drive of claim 1, wherein saidpost-shift sliding distance (d2; m) is the minimum distance between thefirst n-k bits of said TM bit pattern and the last n-k bits of said TMbit pattern-as integer k is varied from 1 to m inclusive.
 11. The diskdrive of claim 1, wherein each track further comprises a postscriptadjacent to and positioned after said TM in each of said servo sectors,the postscript being represented as a postscript bit pattern having atleast n bits.
 12. The disk drive of claim 11, wherein said post-shiftsliding distance (d2; m) is the minimum distance between said TM patternand bits k+1 through n+k of a concatenation of said TM bit patternfollowed by said postscript bit pattern as integer k is varied from 1 tom inclusive.
 13. The disk drive of claim 1, wherein d2 is greater than2.
 14. The disk drive of claim 13, wherein m is greater than
 2. 15. Thedisk drive of claim 1, wherein m is greater than
 2. 16. The disk driveof claim 1, wherein said disk comprises a magnetic disk.
 17. The diskdrive of claim 1, wherein said disk comprises an optical disk.